A fiber optic gyroscope measures angular velocity by comparing the phase difference created between light waves propagated clockwise and counterclockwise around a coil composed of fiber optic cable. The light waves originate from a light source that is coupled to the fiber, split, and directed down a clockwise propagating path and a counterclockwise propagating path. Thereafter, the counter-propagating light waves are recombined and made to impinge upon a photodetector that generates an output signal proportional to the intensity of the combined counterpropagating waves and the phase difference between the counterpropagating waves. The phase difference, and accordingly the angular velocity, is extracted by analyzing the photodetector output signal.
Therefore, for a gyroscope, phase difference is created under two circumstances, when the counterpropagating light waves travel unequal lengths around the fiber cable, and when the cable is rotated about the axis normal to the aperture created by the wound coil. In an ideal gyroscope with symmetrically designed propagating paths, the measurable phase shift between the counter-propagating light waves will directly correspond to the rotation induced phase shift between the counter-propagating waves. Moreover, zero rotation should correspond to zero phase shift between the counter-propagating waves.
Actual gyroscopes are not ideal. Gyroscopes are constructed with non-ideal components that impart errors to the angular velocity measuring capabilities of the gyroscope. Moreover, to affect the most cost-effective fiber gyroscope, it is common to compromise on component choices that can also introduce non-rotation induced phase shift errors into the measuring capabilities of the gyroscope.
Two types of unwanted phase shift errors are traceable to the non-ideal characteristics of gyroscope fiber. Amplitude type phase error and Intensity type phase error are traceable to the mixing of the light wave components from different polarization planes while the light waves traverse the same fiber cable. These phase errors are compounded when single mode type fiber is used to construct the fiber coil of the gyroscope.
Single mode (SM) type fiber is commonly used because it is less expensive than polarization maintaining (PM) type fiber. However, SM fiber is not the optimal fiber choice because SM fiber permits within a fiber cable the concurrent propagation of two or more light waves having the same phase constant in different polarization planes. The inability of the fiber to keep separate the light wave components can lead to both types of phase shift errors. Also, if the polarization planes of the oppositely propagating waves are not aligned when the light waves are recombined, the magnitude of the interference pattern will vary according to the cosine of the acute angle between the oppositely propagating wave polarization planes. Moreover, the high sensitivity of fiber cable to environmental variations such as temperature changes and vibrational stresses augments the problem of multiple polarization planes and unwanted phase shift errors.
Unwanted non-rotation induced phase shift errors can be reduced or eliminated by using a depolarizer in the gyroscope system. By depolarizing the light within a fiber cable, it is more likely that both counterpropagating light waves will encounter the same optical path. A depolarizer will distribute the intensity of each counter propagating light wave into an aggregation of partial lights with polarization planes distributed equally in all directions. Accordingly, the interference pattern developed from ideally depolarized light traversing counter-propagating paths will not vary according to polarization plane differences or misalignment because the interference pattern of the concurrent counter propagating waves will be independent of any respective concurrent counter propagating wave polarization plane.
One type of depolarizer, a Lyot fiber depolarizer comprises coupled segments of PM fiber with the polarization axis of each PM segment aligned at a forty-five degree angle with respect to the polarization axis of the next segment of PM fiber. Additionally, the length of the depolarizer segments should be such that the propagation time of the light polarized along one axis compared to the propagation time of the light polarized along the orthogonal axis is greater than the coherence time of the light propagating therethrough. However, although depolarizers can help to minimize non-rotation induced phase shift errors, depolarizers can also be the source of non-rotation induced phase shift errors. Moreover, environmental variations subjected upon gyroscopes will often augment the problems associated with depolarizers.
In particular, when the design of both propagation paths within a Lyot depolarizer are less than exactly symmetrical, environmental variations can cause unpredictable effects on the depolarizer and ultimately, the measuring capability of the gyroscope. Accordingly, it is desirable to minimize the non-symmetries of gyroscope depolarizers.
To date, the art related to the current invention has not addressed the considerations necessary to design an environmentally robust depolarizer. More particularly, no related art has discussed or suggested the specific design considerations that are suggested by the current inventors to minimize the sensitivity of fiber depolarizers to environmental variations.
For instance, U.S. Pat. No. 5,136,667 issued to Ohno et al., U.S. Pat. No. 5,371,595 issued to Nishiura et al., U.S. Pat. No. 5,347,354, issued to Muller et al., U.S. Pat. No. 5,285,257 issued to Negishi et al., U.S. Pat. No. 5,526,115 issued to Nishiura, and U.S. Pat. No. 5,319,440 issued to Kersey et al., all discuss the use of fiber depolarizer in fiber optic gyroscopes. However, each of these patents merely teaches the use of depolarizers that is ordinary in the art. Moreover, each of these patents specifically does not mention fiber depolarizer design considerations that mitigate the effect of environmental variations.
Finally, U.S. Pat. No. 5,335,064 issued to Nishiura et al (hereinafter "Nishiura") teaches how to make a new type of depolarizer. Nishiura teaches how to make a depolarizer by gluing a polarization maintaining type of fiber segment to the end lead of a polarizer at an angle forty-five degree to the major axis of the polarizer fiber segment. Therefore, Nishiura teaches how to eliminate one segment of PM fiber in making a depolarizer, but does not teach or suggest a need to, or how to, improve the symmetry of a depolarizer, or affect enhanced environmental insensitivity of fiber depolarizers.